The invention pertains to the forced air composting process in which the composting material is stacked in a pile or windrow over an air distribution system such as a plenum or perforated pipes connected to blowers. This process is widely used to convert sewage sludge into beneficial soil conditioner. Microorganisms generate heat as they respire and decompose the organic matter in the pile. Specified minimum temperatures are necessary to meet U.S. Environmental Protection Agency standards for reduction of pathogens, but excessive temperatures inhibit microbial activity resulting in incomplete decomposition of organic matter. Air is either blown upward through the pile, referred to herein as positive aeration, or drawn downward through the pile, referred to herein as negative aeration. The air supplies oxygen to the microorganisms, controls the process temperature by removing excess heat, and dries the compost. Positive and negative aeration each have advantages. Positive aeration is more efficient for and drying of the pile and for providing high temperatures near the surface of the pile. Negative aeration allows higher temperature in the core of the pile and allows capture of the exhaust for treatment. The most well-equipped composting plants provide both aeration modes, referred to as reversible aeration. Periodic reversal of aeration ensures that the temperature and moisture content are uniform throughout the depth of the pile and best ensure that all of the process objectives are met. The invention is a better method for providing reversible aeration capability.
Positive aeration is effected by drawing fresh air into the blower inlet and connecting the composting process air distribution system to the blower outlet. Negative aeration is effected by connecting the composting process air distribution system to the blower inlet. The blower outlet may be connected to an air pollution control device to treat the odorous exhaust gas before discharge.
The most widely used prior art is to connect ductwork to the inlet and outlet of the blower with four separate dampers, slide gates, or valves arranged such that the composting process air distribution system may be connected to either the outlet or the inlet of the blower. This design takes up excessive space and is costly. It cannot be easily automated, because four separate valves must be actuated for reversal of aeration mode.
U.S. Pat. No. 5,540,584 is for a plurality of valves linked mechanically, specifically for air flow reversal in regenerative thermal oxidizers. It is not applicable to the composting process.
A second alternative consists of a box assembly with four ports oriented at 90 degrees. Two opposing ports are connected to the blower inlet and outlet respectively. The other two ports are connected to the air distribution system and the air pollution control device respectively. An inserted plate or rotating damper blade in the assembly can provide positive aeration in one position and negative aeration by being rotated 90 degrees. U.S. Pat. No. 5,692,893 is for such a rotary valve. U.S. Pat. Nos. 3,897,800; 3,985,149; 3,773,066 and 4,842,016 are for valves of various configurations but similar function. None of the aforementioned patents are listed in classes pertaining to composting. With all of these devices, two additional valves are required to close or open the fresh air intake, so the number of valves to be actuated is reduced from four to three, and pressure drop is excessive.
A third alternative is to use manually connected flexible hoses between the blower, air distribution system,and air pollution control device. This alternative cannot be automated. A fourth alternative is to connect the flexible hoses from the blower inlet and outlets to a disk 180 degrees apart and the air distribution system and air pollution control device to a similar disk. The disks are in contact and on a common axis. Rotation of one disk relative to the other effects reversal of the aeration mode. U.S. Pat. No. 5,597,732 is for a small composting apparatus incorporating the rotating disks. It is practical only for small systems, with relatively small hose diameter.
U.S. Pat. No 3,967,779 is for an air mixing valve with a plurality of mechanically linked louvers mounted on parallel shafts inside a single assembly with multiple inlets. This valve does not pertain to air flow reversal.